No owner wants to find it: a lump on the cat's neck or chest with a small, neat hole in the middle, and something moving inside. That is a wolf worm, the larva of a Cuterebra botfly, and the lump around it is called a warble.
It looks alarming, and the urge to squeeze it out is strong. Don't. A larva that bursts or breaks during a home extraction can leave pieces behind that cause lasting inflammation and infection, and in rare cases a severe allergic reaction. Your veterinarian removes it whole, and once that's done, most cats with a skin warble heal well.
What Is a Wolf Worm in Cats?
A wolf worm is the larva of the North American rodent or rabbit botfly, Cuterebra. Rabbits and rodents are the flies' normal hosts. Cats are accidental hosts that pick up the larvae while hunting or nosing around burrows. The Merck Veterinary Manual describes the adult fly as large and bee-like. It doesn't bite or feed.
Three terms come up again and again:
- Warble: the swelling under the skin where the larva lives. People often use the word for the larva too.
- Wolf worm or bot: the larva itself.
- Cuterebriasis: the veterinary name for the infestation.
Warbles are uncommon overall, but most veterinarians in North America are likely to see at least a few cases, mostly in outdoor cats and kittens in summer and fall.
What Does a Wolf Worm Look Like?
A warble is a firm or soft swelling, about 1 centimeter (0.4 inch) across, with a small round opening on top. That opening is the breathing hole. The larva sits under the skin with its rear end at the hole so it can get air, and you may see it move.
Other things owners notice:
- Matted, wet-looking fur over one spot that the cat keeps grooming. This is often the first sign.
- Fluid or pus leaking from the hole.
- Red or irritated skin around the lump.
- A lump on the head, neck or chest. These are the usual places, though a warble can sit on the trunk or belly too.
The larva changes as it grows. Younger larvae are gray to creamy white and up to about 1.5 centimeters (0.6 inch) long. A mature larva is dark, thick, covered in small spines and 3 to 4.5 centimeters (about 1.2 to 1.8 inches) long. That last stage is the one vets most often find, and it is bigger than most people expect.
A bite abscess or a foreign body can look much the same from the outside. If the lump has no hole, our guide to cat cysts and other lumps and bumps covers the other usual suspects. Either way, a new lump is a reason for a vet visit.
How Do Cats Get Wolf Worms?
Cats don't catch wolf worms from other cats. They pick them up where botflies lay their eggs:
- Eggs near burrows. Female flies lay small clusters of eggs around rabbit and rodent burrows, along their runways, and on nearby grass stems, stones and bark.
- Hatching on contact. The eggs hatch in response to the body heat of a passing animal, and the tiny larvae cling to its fur.
- Getting inside. The larvae enter through the mouth or nose, often while the animal grooms, or less often through an open wound. According to the Companion Animal Parasite Council (CAPC), they do not burrow in through intact skin.
- Migration. Over the next weeks the larva travels through the tissues to a spot under the skin.
- The warble. It settles there, opens a breathing hole and grows, usually for about a month and sometimes longer.
- Leaving. The mature larva wriggles out, drops to the ground and turns into a pupa in the soil. In colder regions the pupa can wait out the winter before a new adult fly emerges.
Steps 2 and 3 are why the face matters. A cat investigating a rabbit hole goes in nose first, which puts the eggs right next to its nostrils and mouth.
When Do Cats Get Warbles?
In most of North America, cuterebriasis is a late summer and early fall problem. The Merck Veterinary Manual places skin lesions mostly in summer and autumn, and a review in Today's Veterinary Practice reports most cases in the eastern United States and Ontario from June through October. In warm climates adult flies can lay eggs year-round, so cases there are not limited to one season.
A lump with a hole on an outdoor cat in August or September should put a warble high on the list.
Wolf Worm Symptoms in Cats
Most cats with a warble have only the skin signs above. They may lick or scratch at the spot, and some seem quiet or lethargic.
Less often, the larva wanders into the nose, throat, eye or even the brain or spinal cord instead of settling under the skin. Vets call this aberrant migration, and the signs look very different.
Nose and breathing signs can include:
- Sneezing, sometimes in sudden violent fits
- Nasal discharge, which can be bloody
- Labored or noisy breathing
- Coughing
Nervous system signs can include:
- Blindness
- Wobbliness or lack of coordination
- Seizures
- Head tilt
- Walking in circles
- Sudden behavior changes or dullness
Sneezing can be the early warning. The Merck Veterinary Manual notes that cats with nervous system signs from Cuterebra often had a bout of violent sneezing weeks to months before, and that these cases typically appear between July and September. CAPC adds that nervous system disease is reported more often in cats than in dogs, and that Cuterebra seems to be one cause of feline ischemic encephalopathy, a stroke-like brain injury.
Found a Hole on Your Cat? Don't Squeeze It
Search for wolf worms and many of the top results show people pulling them out at home. That is the step most likely to go wrong.
Don't:
- Squeeze the lump.
- Pull at the larva with tweezers or your fingers.
- Cover the hole, poke anything into it, or try to kill the larva in place.
The Merck Veterinary Manual's guide for cat owners is blunt: the area should not be squeezed, because the larva may rupture. A ruptured larva can lead to infection, a long-running foreign-body reaction in the tissue and, rarely, an anaphylactic (severe allergic) reaction. Abscesses that keep coming back at the same spot can mean pieces were left behind.
Do:
- Call your vet and say you think your cat has a botfly warble.
- Keep the cat indoors until the visit.
- Note where the lump is and when you first saw it.
- Tell the vet whether your cat goes outside and what parasite prevention it gets.
What if the larva already came out? The cat still needs to be seen. The empty pocket under the skin can turn into a deep abscess or skin infection.
How Your Vet Diagnoses and Removes a Wolf Worm
Diagnosis is usually simple. A swelling with a breathing hole and a larva inside is a diagnosis in itself, and finding and identifying the larva confirms it. When a cat has breathing or nervous system signs but no skin warble, the larva is somewhere deeper, and a CT scan or MRI can help find it or the track it left.
The vet usually widens the breathing hole slightly and eases the larva out with fine forceps, taking care to get it out in one piece. Depending on where the warble is and how the cat copes, this is done with the cat awake, sedated or under anesthesia.
Afterward the vet flushes the wound, removes damaged tissue if needed and usually leaves it open to heal from the inside. Your vet may also prescribe pain relief, antibiotics for an infection, or medication to lower the risk of an allergic reaction. Your veterinarian chooses those medicines and doses for your cat.
PetMD puts skin healing at about two to three weeks, and the Merck Veterinary Manual notes it can take weeks to months, with the hair regrowing later still. Many cats bounce back quickly once the larva is gone.
Larvae that have migrated into the nose, eye, brain or spinal cord usually can't simply be pulled out. Treatment relies on medication, supportive care and sometimes surgery, and the outlook is more guarded.
When Is a Wolf Worm an Emergency?
See your vet soon for a single warble on the skin, or a hole where a larva has already come out. It isn't usually an emergency, but book the first appointment you can get.
Call now (your vet or an emergency clinic) if your cat has:
- Seizures, sudden blindness, circling, a head tilt or a sudden change in behavior
- Labored breathing or bloody nasal discharge
- Swelling in or around an eye
- Weakness, collapse or extreme dullness
Some infected cats also develop a body-wide inflammatory reaction called systemic inflammatory response syndrome (SIRS), as the Today's Veterinary Practice review describes. It can make a cat very sick quickly, so a cat that seems unwell overall needs prompt care.
How to Prevent Wolf Worms in Cats
Keeping your cat indoors is the most reliable prevention. The eggs wait around rabbit and rodent burrows, and free-roaming cats are more likely to get warbles than indoor cats. Our look at indoor vs outdoor cats covers the wider tradeoffs.
If your cat does go outside:
- Supervised time or a harness and leash keeps it away from burrows and the dense cover where rabbits and rodents live.
- In summer and fall, run your hands over its head, neck and chest regularly and look for matted fur, a lump or a small hole.
- Ask your vet about parasite prevention. CAPC notes that no product is labeled to prevent Cuterebra. Some broad-spectrum parasite preventives may give partial protection, but the evidence is thin. Our guide to external parasite prevention for cats explains the basics.
Can Humans Get Warbles From Cats?
Not from the cat. CAPC states that a larva in a cat or dog does not pose a health risk to people. Once it has settled under your cat's skin, it can't move on to you.
People can, rarely, pick up Cuterebra larvae the same way cats do, from areas where the eggs are laid, and human cases have been reported in North America. Most myiasis (fly larvae in the skin) diagnosed in the United States, though, comes from other fly species picked up while traveling in the tropics, according to the CDC. If you find a lump with a hole on your own skin, see a doctor.
The Bottom Line
A wolf worm is gross, but on the skin it is usually very treatable. Leave it in and let your vet take it out whole. In an outdoor cat in late summer and fall, treat sneezing fits, breathing trouble or any nervous system sign as a reason to call right away.
Originally written by Dr. Catharina Hjorth. Updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Cuterebra Infestation in Small Animals
- Merck Veterinary Manual (cat owners): Flies and Mosquitoes of Cats
- Companion Animal Parasite Council: Cuterebriasis
- Today's Veterinary Practice: Cuterebriasis in Dogs and Cats (Starkey, 2021)
- Cornell University Shelter Medicine: Cuterebra
- PetMD: Botflies in Cats (Koehler, DVM)
- CDC: About Myiasis
This article is for general information and is not a substitute for advice from your veterinarian.
